log: map --removed to walkopts.force_changelog_traversal...
log: map --removed to walkopts.force_changelog_traversal
This is the flag to forcibly enable the slowpath. I'm not sure if the
slowpath parameter should be merged with this flag, so let's keep it as
an immutable flag for now.
I'll add another flag to support "grep --all-files". These two will be the
flags which aren't directly mapped from the command-line options.